1. Field of the Invention
The present invention relates to a technique of improving output characteristics of a permanent magnets-type synchronous alternating-current motor, and more specifically to a technique of improving output characteristics of magnets containing-type alternating-current motor, wherein a rotor has a rotor core element and a plurality of magnetic pole elements protruded from the rotor core element in the diametral direction of the rotor and each magnetic pole element includes a permanent magnet fitted in a permanent magnet insertion aperture formed along the central axis of the rotor.
2. Description of the Related Art
A permanent magnets-type synchronous motor includes a rotatable rotor, which has a plurality of permanent magnets mounted on an outer circumferential face thereof, and a stator that is fixed to a case to circularly surround the circumference of the rotor and has a plurality of teeth, which three-phase coils are wound on. In such a motor, a flow of control electric current through the three-phase coils of the stator forms a revolving magnetic field, and the rotor rotates by the interaction between the revolving magnetic field and a magnetic field formed by the permanent magnets. A variety of improvements and applications of the permanent magnets-type motor, for example, for the driving source of electric vehicles, have recently been tried to realize a small-sized motor having a large output torque.
A magnets containing-type motor is one of the permanent magnets-type synchronous motors and has permanent magnets embedded in the rotor instead of being applied on the outer circumferential face of the rotor. This structure prevents the permanent magnets from being stripped off by the centrifugal force produced during the rotation of the rotor. The magnets containing-type motor accordingly has the high reliability under the condition of high-speed rotation. In the magnets containing-type motor, the control electric currents are flown through the three-phase coils on the stator to produce a revolving magnetic field. The rotor is revolved through the interaction between the revolving magnetic field and the magnetic field formed by the permanent magnets. For convenience of explanation, the `permanent magnets-type synchronous motor` hereinafter does not include the magnets containing-type motor.
The torque generated in the permanent magnets-type synchronous motor and the magnets containing-type motor depends upon the strength of the revolving magnetic field formed by the three-phase coils wound on the stator, the strength of the magnetic field formed by the permanent magnets, and the loss in the teeth of the stator. The magnitude of the torque is varied by the positional relationship between the teeth of the stator and the permanent magnets at the time of rotation. This results is generating a cogging torque. The cogging torque makes the rotation of the motor unstable and causes the noise and vibration of the motor.
An increase in distance between the teeth of the stator and the permanent magnets of the rotor reduces the cogging torque and ascertains the smooth rotations of the alternating-current motor. The increased distance, however, undesirably lowers the output of the motor.
A variety of techniques have been proposed to reduce the cogging torque by taking into account these problems. The applicant of the present invention has proposed a motor that satisfies Equation (1) given below (JAPANESE PATENT LAID-OPEN GAZETTE No. 2-202329): EQU L=n.multidot.Pp+x.multidot.g+y.multidot.gp+z.multidot.Pp (1)
where L denotes the length of the permanent magnet in the circumferential direction that gives the minimum cogging torque, g denotes the interval between the rotor and the stator, gp denotes the gap between the adjoining teeth of the stator, Pp denotes the teeth pitch, n denotes an arbitrary natural number, x denotes a real number satisfying 0.4.ltoreq.x.ltoreq.0.6, y denotes a real number satisfying 0.3.ltoreq.y.ltoreq.0.5, and z denotes a real number satisfying -0.06.ltoreq.z.ltoreq.-0.04.
In a proposed magnets containing-type motor with the permanent magnets embedded in the circular iron core, the residual magnetic flux density of the permanent magnets is used to determine the dimensions between the permanent magnets, in order to reduce the vibration and the noise due to the cogging torque (JAPANESE PATENT LAID-OPEN GAZETTE No. 7-79536).
The cogging torque is significantly affected by the magnetic flux density of the clearance between the rotor and the stator. These techniques are accordingly not applicable to the magnets containing-type motor having the magnetic pole elements that are protruded substantially in the diametral direction from the rotor core element.
FIGS. 12A and 12B show examples of the computer-aided analysis for determining the magnitude of the cogging torque. In the graphs of FIGS. 12A and 12B, L1 represents the central angle of the arc of the permanent magnet (hereinafter referred to as the `magnet opening angle`) and Pp represents the angular pitch of the teeth of the stator. The graph of FIG. 12A shows the relationship between the magnet opening angle L1 and the cogging torque in the case of t1=t2=0, where t1 denotes the interval between the side face of the magnetic pole element and the permanent magnet, t2 denotes the interval between the circumferential face of the magnetic pole element and the permanent magnet, and g denotes the gap or the interval between the inner circumferential face of the stator and the outer circumferential face of the magnetic pole element. In the case of FIG. 12A, there are no magnetic pole elements and the permanent magnets are directly applied on the outer circumferential face of the rotor. Ranges A1 and A2 in FIGS. 12A show the magnet opening angles that give the minimum cogging torque, which are determined by the technique described in JAPANESE PATENT LAID-OPEN GAZETTE No. 2-202329. As shown in FIG. 12A, in the permanent magnets-type synchronous motor, the cogging torque is minimized in the ranges of the magnet opening angle L1 determined by the technique described in the above-mentioned gazette.
FIG. 12B shows the relationship between the magnet opening angle L1 and the cogging torque in the case of t1=4 g and t2=2 g, that is, in the case where the permanent magnets are fitted in the magnetic pole elements of predetermined dimensions in the magnets containing-type motor. As shown in FIG. 12B, the magnet opening angles L1 that give the minimum cogging torque are greater than the ranges A1 and A2 of the magnet opening angle L1 determined by the technique described in the above-mentioned gazette. This means that the magnet opening angles L1 determined by the technique described in the above-mentioned gazette can not minimize the cogging torque in the magnets containing-type motor.
It is necessary to minimize the cogging torque while satisfying a variety of requirements, such as the maximum torque and the power consumption, in the process of designing the motor. It is possible to change the variety of parameters and determine the magnet opening angle that gives the minimum cogging torque based on the computer-aided analysis, every time when the magnets containing-type motor is designed. Such analysis, however, causes an extremely large design load.